Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, Acquired During Period		$ 74.9	$ 102.0
Change in goodwill	$ 0.1
Property, Plant and Equipment, Net	306.0	306.0	245.2
Goodwill [Roll Forward]
Net carrying value at beginning of period		546.7	444.7
Net carrying value at end of period	$ 621.6	621.6	$ 546.7
Inland Northwest Bank [Member]
Goodwill [Line Items]
Change in goodwill		$ 0.3